Prompt and accurate locating of buried optical fiber cable for maintenance and repair is essential for minimizing disruption in telecommunication service. The two basic types of optical fiber cables presently used in the industry are metallic cables and dielectric (or non-metallic) cables. At present, it is believed that slightly less than one-third of telecommunication systems use dielectric fiber optic cable. However, surveys indicate that many, if not most, would use dielectric cables instead if there existed a reliable and cost-effective method to locate the dielectric cable after it has been buried.
Presently, there are basically two general types of technologies involved in the detection of buried fiber optic cable, magnetic and metallic. In general, the existing magnetic locators consist of either 1) permanent magnetic elements designed or embedded into the cable, or 2) a permanent magnetic-field emitting product which is buried alongside the length of the cable. With these two magnetic-based detection techniques, the residual magnetization generated by the magnetic hysteresis of the permanent magnetic material within the cable may then be detected by a magnetic field sensor. However, due to the inconsistencies in the distribution pattern of the magnetic field, it is often difficult to detect the correct cable when other magnetic members are located nearby. In an additional arrangement, the cable may be positioned within a separate duct which is magnetized.
Alternatively, there exists fiber optic cable locators which are capable of locating cables which were specifically designed to include metallic components, such as a protective sheathing member or a strength member. Such detection methods detect the electromagnetic field emanating from the metal as a result of the application of an alternating current or an alternating magnetic field to the metallic sheathed cable. However, the electric and/or magnetic field generated by the application of an AC electric signal to the cable is often not strong enough to allow a determination of the precise location of the cable. Ineffective low levels of field strength are of a particular interest when attempting to locating cables where the AC signal is transmitted from a remote position and must travel a substantial distance. Additionally, these metallic sheaths are susceptible to damage due to lighting strikes or corrosion.
Commonly, to facilitate detecting a dielectric cable, a copper ground wire is positioned just above the cable. However, the exposed nature of this ground wire makes it very vulnerable to lightning strikes and propagation of the damages to the optical fibers in the cable. Furthermore, when a cable's armor or detection wire is severed by lightning or some other cause, it becomes impossible to apply an electric signal along the cable, thus creating great difficulty in locating and retrieving the cable for repair. Therefore, it is preferable to employ the locating marker inside the cable to ensure locating accuracy.
Other existing detection methods include incorporating a non-conductive tape which has been covered with a magnetic powder such as strontium or barium ferrite or compounding and extruding these magnetic powders in polyethylene or polyvinyl chloride conduits. Both of these methods employ a means of magnetizing a tape and helically wrapping it along the length of the cable or magnetizing a strip along the length of the conduit. Each method also provides a distinctive electronic-detection signature which allows an operator to differentiate between a buried cable and a solid metallic pipe. See U.S. Pat. Nos. 5,006,806 and 5,017,873 which are assigned to Schondstedt Instrumentation Company of Reston, Va. One other concern with existing techniques is an inability to accurately position and secure the locatable particles as needed within the cable without decreasing the line speed at which the cables can be produced. This is of particular interest in configurations where the locatable particles are placed within portions of the cable which are often extruded, such as the outer jacket.
What is needed and seemingly not available in the prior art is a system which dependably, accurately and cost-effectively locates dielectric (non-metallic) buried cables. Also desirable is a system for locating buried dielectric cables which is readily adaptable to most, if not all, existing cable types. One newly introduced method involves modifying the existing water-blocking tape present in the cable so that the cable becomes magnetically locatable without adversely affecting the operational characteristics of the cable. Such a design is described in commonly assigned U.S. Pat. No. 5,305,410. Additionally, a second commonly assigned U.S. Pat. No. 5,305,411 which issued Apr. 19, 1994, discloses introducing magnetic particles into the existing strength members utilized within most cable designs. Yet another cable design is described in a commonly assigned application filed on the same date as this application. However, while addressing concerns similar to that of the above-identified co-pending identified co-pending applications, the present invention is not directed toward modifying the strength member portion or the water-blocking portion present in most communication cables as taught by the applications identified immediately above, but instead, introduces a variety of different arrangements regarding the orientation of the particular magnetic particles within the cable to optimize the ability of an operator to precisely locate an all-dielectric cable using a magnetometer after the cable has been buried.